Earthquake Report: Alaska Peninsula!

Below is my interpretive map where I use Google Earth and the kml (keyhole markup language) files from the USGS to plot epicenters, Modified Mercalli Intensity Scale (MMI) contours, and the subduction zone slab contours for this region (Hayes et al., 2012). I plot the USGS moment tensor from this M 6.2 earthquake, along with moment tensors for earthquakes further west along the Aleutian Trench (see below).

The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.

The red-orange-yellow lines are slab contour lines from Hayes et al. (2012). These lines are a best estimate for the depth to the subduction zone fault. These are based largely upon seismicity and there is currently an effort to update these contours to integrate other data types. This M 6.2 earthquake is too shallow to be on the megathrust, so it is clearly a crustal earthquake in the North America plate. The USGS depth is 17.9 km, but the slab depth is between 80 and 100 km at this location (the slab contours have 20 km intervals).

There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

I include some inset figures.

In the upper left corner, I place a map created by Dr. Peter Haeussler, USGS, which shows the historic earthquakes along the Alaska and Aleutian subduction zones. I place the epicenter from today’s earthquake as a red star. Note that this earthquake is near the possible segment boundary shown as the boundary between the 1938 and 1964 earthquakes.

To the right of Hauessler’s map is a figure from Atwater et al. (2005) that shows the earthquake cycle and how a tsunami can be generated at a subduction zone.

Below the Atwater figure I show a cross section of a subduction zone through the two main parts of the earthquake cycle. The interseismic part (in-between earthquakes) and the coseismic part (during earthquakes). This was developed by George Plafker and published in his 1972 paper on the Good Friday Earthquake.

Here is an animation that shows earthquakes of magnitude > 6.5 for the period from 1900-2016. Above is a map showing the region and below is the animation. This is the URL for the USGS query that I used to make this animation in Google Earth.

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Status & Contact Info

Jay Patton is currently living in Arcata, CA. He was most recently teaching classes at Humboldt State University, Dept. of Geology. and is available for geological consulting.
HSU Dept. Geology website twitter: @patton_cascadia
Contact him at:
quakejay at gmail.com

Chicken the Cat

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